This invention relates to signal processors for pulse doppler radars, and more specifically to such processors providing Moving Target Indicator (MTI) operation and Constant False Alarm Ratio (CFAR) capability.
One of the common problems with conventional moving target indicator (MTI) processing is its inability to completely eliminate false alarms. This problem has drawn increased attention of late because of the disabling effects which excessive false alarms can have an automatic detection and tracking systems. Even well designed, coherent MTIs, capable of providing 40dB or more of signal-to-clutter ratio improvement, suffer in this regard. The problem is due, in major part, to very large clutter scatterers such as water towers, cliff faces and the like. Even after suppression by MTI processing, echoes from such scatterers often are strong enough to exceed detection thresholds.
Approaches to solution of this problem commonly have involved some form of automatic gain control (AGC) or normalization which attenuates very strong input signals to a level such that the MTI suppression will suffice to prevent false alarms. Probably the most extreme example of such prior approaches is the "hard limited" MTI processor, in which the processor chain comprises a hard limiter followed by MTI and pulse compression.
The hard limiter output contains only phase information and is independent of the level of its input. Hence, low-doppler returns of all amplitudes will be suppressed by the MTI to a well defined level below a detection threshold. In-the-clear target returns, on the other hand, will not be suppressed by the MTI, and will be detected after integration via pulse compression.
Thus, the hard limited MTI does provide the desired CFAR capability as well as an "in-the-clear" target detection capability. Unfortunately, it does not also provide a capability to detect targets immersed in clutter. Such target returns are suppressed, at the hard limiter output, according to the clutter-to-signal ratio at its input, and the pulse compression integration will be inadequate for reliable detectability.
The hard limited MTI accordingly provides CFAR and intra-clutter visibility, but fails to provide adequate subclutter visibility. In very patchy or spiky clutter, intra-clutter visibility will permit automatic target tracking even though returns are occasionally lost in the clutter. However, in heavier, more homogeneous clutter situations, subclutter visibility, which is attainable only by linearly processing the signal-plus-clutter return, is required.
Another known approach, which achieves CFAR operation without the attendant loss of subclutter visibility, involves an AGC function incorporated prior to the MTI. The attenuation as a function of range must, of course, be periodic so as not to destroy the cancellation. This approach, however, suffers from a serious shortcoming (beyond the practical difficulties associated with generating a periodic attenuation function) when employed with coded pulses, especially those with high time-bandwidth products.
The AGC circuit precedes the MTI which, in turn, precedes the pulse compression network in order to limit the required dynamic range of the latter. Hence, the attenuation which accompanies the return from a large clutter scatterer lasts for at least the uncompressed signal pulse width. This means that any signals of interest, separated from the strong clutter scatterer by something less than the range extent of the uncompressed pulse, will be quieted by the presence of the clutter. This phenomenon can seriously degrade the detection capability of the radar.